A gate valve, also known as a slab valve, is a valve for use in pipelines and the like, which opens and shuts by laterally translating a “gate” out of and into the path of the fluid in the pipeline. Typically, within a valve body structure a gate member is movable by a linearly-movable valve stem between an open position, where flow of fluid through inlet and outlet passages is permitted, and a closed position where the gate member blocks the flow of fluid through the inlet and outlet passages. In a “full body” design, the gate member may define a flow port that is aligned with the flow passages in the open position of the gate and segment assembly. In compact or “short body” gate valves, the gate member may be non-ported, in which case it is opened by the actuating stem of the valve to an open position where it is retracted to a position within the bonnet structure of the valve body and substantially clear of the flow passages.
Gate valves are primarily used to permit or prevent the flow of liquids. Gate valves are often used when a straight-line flow of fluid and minimum restriction is desired. Gate valves are typically designed to be operated in the fully opened and fully closed positions, and are not typically used for regulating flow. When fully open, a typical gate valve has no obstruction in the flow path, resulting in very low friction loss and pressure drop. The sealing faces on the gate can form a wedge shape or they can be parallel. Gate valves may have flanged ends that are drilled according to pipeline compatible flange dimensional standards. Gate valves are typically constructed from steel and cast iron.
Typical gate valves shut off flow in a pipeline by sealing against the low-pressure “downstream” side of the valve housing the force generated by the fluid in the high-pressure “upstream” side of the valve. As the gate is moved into the flow path of the fluid, the moving fluid pushes the gate against the downstream side of the housing, covering up and sealing the downstream flow path. This inherent functionality of typical gate valves has many drawbacks. First, to the extent the valve relies on pressure from one direction to seal, loss of pressure in that direction or gains in pressure from the other direction in the pipeline may cause the valve to lose its seal. Second, the functionality of the seals in a typical gate valve cannot be tested onsite without opening or operating the entire pipeline system to see if shutting off a particular valve shuts down the flow in the system. This can be time consuming, expensive, or impractical in large installations with a number of valves.
To address some of the problems with typical gate valves there have been multiple attempts to create “expanding” gate valves that do not rely on upstream pressure to affect a seal. Expanding gate valves have employed various mechanisms that tend to expand the thickness of the gate when it is translated into its fully shut position, thereby theoretically causing the gate to seal against both the upstream and downstream sides of the valve body using only the mechanical force of shutting the valve. Theoretically, once an expanding gate valve is fully expanded and sealed in the shut position, then both the low and high pressure sides of the pipeline are blocked, and then the pressure inside the valve body can be bled-off. This safely isolates the upstream and downstream pipes with redundant seals. Also, the pressure inside the valve body can be remotely monitored after the bleeding to detect any subsequent seal failures. This desirable functionality is called “double block and bleed.”
While multiple attempts have been made to achieve a well-functioning expanding gate valve, each of the prior designs has important real-world drawbacks. By way of example and not limitation, each of the previously-known expanding gate valves exhibit one or more of the problems: they were larger than typical gate valves and thus could not fit into existing pipelines as a direct replacement for regular gate valves; they would not apply consistent force across the face of the seal, which leads to leaks and binding; they would tend to seal against pressure better in one direction than the other; they would forcefully drag the face of seal across the sealing surface during engagement and/or disengagement, which tends to damage the seal; they required a large amount of force to actuate, requiring larger and more expensive actuators; they would tend to get “stuck” in place once actuated; they would not expand and seal in both the open and closed positions; they would not operate smoothly and consistently; when open, they did not provide a consistent inner surface with the pipeline, which creates pressure drops and interferes with cleaning “pigs” that are run through the lines; they would be complicated and expensive to manufacture; they required special tools to service; and they would be prone to failure. An improved expanding gate valve with double-block-and-bleed functionality that overcomes these drawbacks has been needed for many years.